This invention relates to diffractive spectral filters that are particularly suited for use with condensers and, particularly, condenser that collects radiation, here soft x-rays, from either a small, incoherent source and couples it to a camera designed for projection lithography.
Projection lithography is a powerful and essential tool for microelectronics processing. As feature sizes are driven smaller and smaller, optical systems are approaching their limits caused by the wavelengths of the optical radiation. xe2x80x9cLongxe2x80x9d or xe2x80x9csoftxe2x80x9d x-rays (a.k.a. Extreme UV) (wavelength range of xcex=100 to 200 xc3x85) are now at the forefront of research in efforts to achieve the smaller desired feature sizes.
In photolithography, a condenser optical system couples radiation from a source to illuminate a mask plane which is relayed onto a wafer by an imaging system. Sources of radiation typically generate a continuous spectrum of radiation whereas the radiation of interest is EUV. Condensers typically employ a spectral purity filter or window made of silicon, beryllium, zirconium or other suitable transmitting material to let radiation of the desired wavelength(s), e.g., 13.4 nm, pass from the radiation source to reach the reticle mask. Unfortunately, 1 xcexcm thick silicon window, for example, can absorb about half of the 13.4 nm radiation entering the window; a corollary is that the silicon window can become overheated. The art is still in search of techniques to improve spectral filtration efficiency and thermal management.
The present invention is directed to a condenser which employs a diffractive grating that functions as a spectral purity filter to eliminate out-of-band radiation from being projected onto a reflective reticle mask from the condenser.
The invention is based in part on the recognition that an input beam of radiation projected onto the diffractive grating will generate different orders of radiation. By properly designing the diffractive grating, the diffractive spectral filter functions to allow desired radiation to pass to the reticle mask without the attendant high level of waste associated with spectral filters that employ transmission windows.
Diffractive spectral filters offer a potentially crucial advantage in thermal management. Transmission windows are thin membranes that cannot be actively cooled. At commercial-tool flux levels, the membranes may overheat to the point of failure. Diffractive spectral filters are preferably fabricated on thick substrates that can be actively cooled like other condenser elements.
A preferred condenser employing the diffractive spectral filter includes a series of aspheric mirrors on one side of a small, incoherent source of radiation producing a series of beams. Each aspheric mirror images the quasi point source into a curved line segment. A relatively small arc of the ring image is needed by the camera; all of the beams are so manipulated that they all fall onto this same arc needed by the camera. Also, all of the beams are aimed through the camera""s virtual entrance pupil. The invention is based in part on the recognition that improvement in overall system efficiency is achieved by including a correcting mirror for reshaping a beam segment. The improved condenser efficiently fills the larger radius ringfield created by today""s advanced camera designs.
Accordingly, in one embodiment, the invention is directed to a condenser system for use with a ringfield camera that includes:
a small compact source of continuous spectrum of radiation light;
collector mirrors comprising at least two substantially equal radial segments of a parent aspheric mirror, each having one focus at the radiation source and a curved line focus filling the object field of the camera at the radius of the ringfield and each producing a beam of radiation;
a corresponding number of sets of correcting mirror means which are capable of translation or rotation, or both, such that all of the beams of radiation pass through the entrance pupil of the camera and form a coincident arc image at the ringfield radius, wherein at least one of the correcting mirrors of each set, or a mirror that is common to said sets of mirrors, from which the radiation emanates, is a concave relay mirror that is positioned to shape a beam segment having a chord angle of about 25 to 85 degrees into a second beam segment having a chord angle of about 0 to 60 degrees; and
one or more diffractive spectral filters for separating first radiation light having a particular wavelength from the continuous spectrum of radiation light.
In another embodiment, the invention is directed to a condenser system for use with a ringfield camera that includes:
a small compact source of continuous spectrum of radiation light;
collector mirrors comprising at least two substantially equal radial segments of a parent aspheric mirror, each having one focus at the radiation source and a curved line focus filling the object field of the camera at the radius of the ringfield and each producing a beam of radiation;
a corresponding number of sets of correcting mirror means which are capable of translation or rotation, or both, such that all of the beams of radiation pass through the entrance pupil of the camera and form a coincident arc image at the ringfield radius, wherein at least one of the correcting mirrors of each set, or a mirror that is common to said sets of mirrors, from which the radiation emanates, is a concave relay mirror that is positioned to shape a beam segment having a chord angle of about 25 to 85 degrees into a second beam segment having a chord angle of about 0 to 60 degrees; and
wherein the one or more of the corresponding number of sets of correcting mirrors means includes diffractive spectral filter means for separating first radiation light having a particular wavelength from the continuous spectrum of radiation light.
In a preferred embodiment, the concave mirror comprises six substantially equal radial segments of a parent aspheric mirror. In another preferred embodiment, the concave mirror shapes a beam having a chord angle of about 25 to 85 degrees and preferably at about 50 degrees into a second beam having a chord angle of about 0 to 60 degrees and preferably at about 28 degrees.
In another embodiment, the invention is directed to a process for fabricating integrated devices that includes at least one element having a dimension xe2x89xa60.25 xcexcm and preferably xe2x89xa60.18 xcexcm by projection lithography that employs the inventive condenser system.